Non-destructive sensing techniques using electromagnetic waves in a frequency region from the millimeter wave band to the terahertz wave band (more than 30 GHz and not more than 30 THz) have been developed. Fields of application of electromagnetic waves of the frequency bands cover imaging techniques using safe fluoroscopic examination apparatus that replace X-ray apparatus. Techniques such as a spectroscopic technique of examining physical properties of a substance such as the state of bonding by determining the absorption spectrum and/or the complex dielectric constant in the inside of the substance, a biomolecular analysis technique and a technique of evaluating a carrier concentration and mobility have been developed. Additionally, development of examination apparatus for examining the presence or absence of a substance showing an absorption spectrum specific to the terahertz band, or a so-called fingerprint spectrum, is being discussed. Such an examination apparatus can operate for high-speed examination when it is discretely provided with oscillators having respective oscillation frequencies (typically from 0.1 THz to 10 THz) near the fingerprint spectrum of the substance to be examined because it does not involve any sweep in the time domain or the frequency domain.
Means for generating a terahertz wave include those adapted to generate a pulse wave by irradiating a photoconductive element with light from a femtosecond laser and those for parametric oscillations that are adapted to generate a wave of a specific frequency by irradiating a non-linear crystal with light from a nanosecond laser. However, all such means are based on light excitation and face limits for downsizing and reduction of power consumption. Thus, structures using a quantum cascade laser or resonant tunneling diode (RTD) as current injection type element for operating in the region of terahertz waves are being discussed. Particularly, research efforts are being paid on the latter, or resonant tunneling diode type elements, as they operate near 1 THz at room temperature (see Patent Literature (PTL) 1 and Non-Patent Literature (NPL) 1). Such elements are typically formed by using quantum wells including GaAs/AlGaAs or InGaAs/InAlAs produced by way of lattice-matching-based epitaxial growth on GaAs or InP substrate. The element oscillates as the voltage is biased near the negative resistance region of the voltage/current (V-I) characteristic as illustrated in FIG. 5. A flat antenna structure formed on the substrate as illustrated in PTL 1 is employed as resonator structure for oscillation.
Such an RTD element shows a gain over a wide frequency region. Therefore, it is necessary to suppress the parasitic oscillation attributable to resonance points of relatively low frequencies other than the desired oscillation that is generated as a result of connecting a power bias circuit to the RTD element. The parasitic oscillation is suppressed by connecting a resistor in PTL 1, or a diode element 63 in NPL 1 as illustrated in FIG. 6 in parallel with an RTD element 64. Note that, in FIG. 6, 60 denotes a transmission line that also operates as slot antenna for taking out the oscillation output and 61 and 62 denote the capacity elements at the terminal sections of the transmission line. An oscillator is formed by 60, 61, 62 and 64. 65 denotes a power source (Vbias) for applying a voltage to the RTD element 64 and 66 shows the sum (Rbias) of the internal resistance of the power source 65 and the resistance that connection line 67 has. A power bias circuit is formed by 65, 66 and 67.